to remove the maintenance page you can edit the ow_config table in the database and fine the maintenance bit, change it from "1" to "0" and it should kill maint mode. Most likely since updating a plugin needs to update the database and has failed. the site will go into maint mode each time you try to do the update.
I had this a while back and could not do anything in admin area because a plugin db update had locked up. It took Addenster to jump in and free it up.